Method for the molten salt electrolytic production of metals from metal chlorides and electrolyzer for carrying out the method

ABSTRACT

There is provided a new method and electrolyzer design, especially for the electrolysis of MgCl2. Hollow cathodes having ducts therein provide communication between a metal collection zone and an electrolysis zone, or a gas separation zone to be more accurate. The &#39;&#39;&#39;&#39;window opening,&#39;&#39;&#39;&#39; at electrolyte surface level, in the partition wall between the metal collection and electrolysis zones is eliminated to avoid the disadvantageous flow pattern associated therewith.

United States Patent [1 1 Andreassen et al.

[ Sept. 23, 1975 METHOD FOR THE MOLTEN SALT ELECTROLYTIC PRODUCTION OF METALS FROM METAL CHLORIDES AND ELECTROLYZER FOR CARRYING OUT THE METHOD [75] Inventors: Knut Anton Andreassen; Kjell Bj'drn Stiansen, both of Porsgrunn, Norway [73] Assignee: Norsk Hydro A.S., Oslo, Norway [22] Filed: Jan. 29, 1974 [21] Appl. No.: 437,608

[30] Foreign Application Priority Data Jan. 30, 1973 Norway 371/73 [52] US. Cl 204/70; 204/237; 204/243 R;

[51] Int. Cl. C22d 3/08; C22d 3/02;B01k 3/04 [58] Field of Search 204/68-70, 204/243 R-247, 237, 284

[56] References Cited UNITED STATES PATENTS 2,393,686 l/1946 Hunt et al 204/70 Meier et al. 204/246 Vladimirovich et al. 204/244 X FOREIGN PATENTS OR APPLICATIONS 895,502 3/1972 Canada 204/244 Primary Examiner,lohn H. Mack Assistant Examiner-D. R, Valentine Attorney, Agent, or FirmWenderoth, Lind & Ponack [57] ABSTRACT There is provided a new method and electrolyzer design, especially for the electrolysis of MgCl Hollow cathodes having ducts therein provide communication between a metal collection zone and an electrolysis zone, or a gas separation zone to be more accurate. The window opening," at electrolyte surface level, in the partition wall between the metal collection and electrolysis zones is eliminated to avoid the disadvantageous flow pattern associated therewith.

9 Claims, 3' Drawing Figures US Patent Sept. 23,1975 Sheet 2 of2 3,907,651

METHOD FOR THE MOLTEN SALT ELECTROLYTIC PRODUCTION OF METALS FROM METAL CHLORIDES AND ELECTROLYZER FOR CARRYING OUT THE METHOD BACKGROUND OF THE INVENTION This invention relates to a method for the molten salt electrolytic production of metals from metal chlorides, and more particularly a method of producing magnesium and chlorine from molten salt containing magnesium chloride. Further, the invention relates to an electrolyzer having double-acting cathodes for carrying out the method.

In the ordinary production of magnesium by molten salt electrolysis of magnesium chloride, the electrolyte circulates in a closed loop between the electrolyzing zone and what may be called the outside-lying zones. This natural circulation is probably essentially due to the gas lift effect. One of the main requirements for carrying out the magnesium chloride electrolysis and for the constructional design of Mg electrolysis cells therefor is that the two products, magnesium and chlorine, should be collectable with a minimum of loss. Since recombination of the products is an obvious source of possible loss, it is considered highly important to prevent this by aiming at rapid and complete separation of magnesium and chlorine. Usually the gas is first removed from the electrolyte/metal mixture in a gas separation zone and the metal is then removed from the electrolyte in a metal separation zone.

In the classical IG-electrolyzers for magnesium chloride electrolysis, the chlorine gas and metal produced are maintained separate by means of special curtains, so-called diaphragms, which divide each cell into zones for collecting chlorine and magnesium, respectively. Therefore, an electrolyzer having a plurality of cells has a number of gas and metal collecting zones.

However, the development has taken a direction towards larger and more efficient types of electrolyzers having double-acting cathodes, wherein the gas from the cells is collected in a common zone and wherein the natural flow is utilized also to transport metal and electrolyte from each cell to a commmon separation and collection zone for metal. Electrolyzers which work according to this principle comprise three main zones, the electrolysis zone, gas separation zone and metal separation zone. The term gas separation zone is herein intended to mean the zone at the cathode top or immediately above the cathodes. In this zone, the upward flow of molten salt is drastically deflected, as further described below. The two first-mentioned zones are in principle separated from the last-mentioned zone by a partition wall. This partition wall has openings at its lower end to allow the electrolyte to flow into the electrolysis zone. Further, openings are provided at the upper end, although below the surface of the electrolyte, through which openings electrolyte and magnesium metal formed can flow from the gas separation zones into the metal collection zone. An electrolyzer of this type is shown and described in Norwegian Pat. No.

Characteristic of all versions of this known electrolyzer design having a common metal separation zone for a number of cells and a unified transportation of electrolyte and metal thereto, is that the communication between the gas separation zone and the metal separation zone takes place via window openings, at electrolyte surface level, in the partition wall therebetween. This solution has a number of obvious weak nesses.

Metal which is produced at sites remote from the opening will have a relatively long residence time in an electrolyte full of gas with a consequent increased risk of recombination. The main flow of electrolyte in the electrolysis zone is diagonal so that the gas is to a great extent moved towards the partition wall and can readily follow the electrolyte through the opening. There is small possibility of controlling the velocity of the electrolyte in front of and in the opening so that it is difficult to achieve a sufficient difference between the gas bubble and electrolyte velocity vectors and a resulting efficient separation of gas from electrolyte and metal.

This known main principle of electrolyzer design complicates the weighing of a desirable shortest possible residence time for the metal in the electrolysis and gas separation zones against a desirable minimum of transfer of gas into the metal separation zone.

Attempts have been made to solve these problems in various ways. For example, it has been proposed to use increasing distance between the electrodes in the direction of the partition wall, decreasing electrode height in that same direction and partial suction removal of chlorine through the anodes, a greater amount being then removed near the partition wall.

Further, it is known to allow the metal to separate out in each cell and then allow the metal to be transferred via inverted troughs to a common metal collection zone. The obvious advantages in having few zones for gas and metal collection are utilized also in this case. An electrolyzer based on this principle is shown and described in Norwegian Pat. No. 87,636.

In this cell construction, gas and metal separation zones are practically speaking identical or coinciding. This limits the possibilities of achieving a great difference in gas bubble, metal droplet and electrolyte velocity vectors in the critical regions. Therefore, again the weighing of a shortest possible residence time for the metal in the electrolysis and gas separation zones against the least possible transfer of gas to the metal collection zone is very complicated. Even with an optimal adjustment of the parameters in question, the resulting flow which can be provided in this known construction will no doubt lead to a considerable loss of gas.

SUMMARY OF THE INVENTION The purpose of the present invention is to effect an accumulation of the two main products magnesium and chlorine in a low number of zones with a minimum of loss due to recombination thereof, providing a rapid and complete separation of gas and metal. More particularly then, a purpose of the invention is to provide a method rendering an extensive possibility of varying flow velocities in the critical region in which gas separation is to take place.

Other advantages and purposes of the invention will be apparent from the following description.

According to the invention it is contemplated to realize the main purpose thereof by allowing the electrolyte and metal to flow in a direction substantially opposite to the direction of the gas bubbles during the gas separation so that the velocity vectors in question will thereby be as different as possible. This method is characterized in that the metal together with the electrolyte are separated from the rising stream of chlorine gas by being deflected in a direction substantially opposite to the direction of the rising gas bubbles, the metal and electrolyte being made to flow, utilizing the natural convection, through openings or ducts provided in each cathode, such duets terminating at or in the adjacent metal separation or collection zone, whereupon the electrolyte continues to circulate into the electrolysis zones through openings provided near the bottom of the clectrolyzer, the separated metal rising to the surface of the metal collection zone, as known per se.

The invention also relates to an clectrolyzer for carrying out the method and having double-acting cathodes and a partition wall between the electrolysis and gas separation zones on one side and the metal separation zone on the other, there being provided in each cathode one or more ducts which provide communication between gas separation zones and the metal separation zone.

The inlet openings of the ducts are preferably located in the horizontal plane of the cathode tops, the outlet openings being located in the vertical plane of the partition wall.

The method of the invention results in that the flow in the electrolysis zone, that is the space between anode and cathode, will be substantially in a vertically upward direction. There is no tendency to produce a diagonal flow. Basically this is the most correct flow pattern, that which will remove gas and metal from that zone in a minimum of time. In the region above the cathode, which here constitutes the gas separation zone, the electrolyte and metal flow make a 180 turn and continue vertically downward. Basically this is the best solution in order to rapidly and completely remove gas from the liquid. The electrolyte and metal flow then continues within the cathode towards the metal separation zone without interfering with the conditions in the electrolysis and gas separation zone.

The choice of site and shape of the inlet opening of the duct enables a high degree of control of flow velocities in the gas separation zone to be achieved. By varying the width of the cathode top and of the inlet slit the velocities can be adjusted so that the metal will quickly be removed from the gas separation zone by the electrolyte while the gas is retained. Further, the construction results in that the whole region above the cathode functions as a gas separation zone as contrasted to cells having the usual window opening. Thus, there is no need for provisions such as for instance decreasing electrode height towards the partition wall or increasing the degasification area in the proximity thereof.

The rapid and efficient separation of gas from electrolyte/metal mixture achieved by the invention provides a number of possible advantages.

Decreased contact time between gas and metal reduces the degree of recombination and thereby in-.

creases the current efficiency of the electrolysis.

Reduced transfer of gas into the metal collection zone means a lower actual loss of gas and reduces pollution problems caused by such loss of gas.

Improved current efficiency as well as decreased loss of gas each represents cost reductions.

BRIEF DESCRIPTION OF THE DRAWINGS In thefollowing, the invention will be described in further detail in connection with an embodiment of an clectrolyzer which is particularly suitable for effecting an electrolyte circulation according to the invention and which is shown in the accompanying drawings, wherein FIG. 1 is a horizontal section of an clectrolyzer having double-acting cathodes according to the invention.

FIG. 2 is a vertical section along the line AA in FIG. 1.

FIG. 3 is a vertical section along the line B-B in FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION Anodes l and cathodes 2 are arranged alternately so that both electrode types are double-acting. The electrolysis zone is shown by designation numeral 3, the gas separation zone by 4 and the metal separation zone by 5. The duct 6 in the hollow cathode has an inlet at 7 and an outlet at 8. The inlet opening of the duct is located in the horizontal plane of the cathode top, while the outlet opening is located in the vertical plane of the partition wall 10. The cathode is supported by supporting plate 9. A sloping top edge of the cathodes 2 is indicated at 11 where the cathodes extend under the partition 10, and at 12, the internal cathode bottom is showed to be sloping.

The flow pattern of the coil construction is indicated by arrows. Mainly as a result of the evolution of gas at the anode, the electrolyte flows, together with the gas and metal formed at the cathode, vertically upward in the electrolysis zone 3. In the gas separation zone 4, the electrolyte/metal flow makes a 180 turn to flow vertically downward into the duct 6 provided in the cathode 2. By selecting the appropriate dimensions of the cathode top and the inlet opening of the duct, it is achieved, as mentioned, that the gas will practically speaking leave the liquid completely while the metal will follow the flow of electrolyte. Within the cathode the electrolyte/metal flow makes a 90 turn before entering into the metal separation zone 5. In the latter, the metal separates out, rises to the surface of the electrolyte to remain there. The electrolyte flows downward, turns to flow under the electrodes and from there returns to the electrolysis zone 3.

It will be understood that the elec'trolyzer shown in FIGS. 1-3 only represents a preferred embodiment of an eleetrolyser to be used for carrying out the method of the invention in practice and that other constructions and modifications can be employed which provide a suitable relationship of the parameters having an influence upon the flow pattern. For instance, the cathodes can be provided with a number of ducts and their inlet openings as well as their outlet openings at the metal separation zone can be varied to suit the flow pattern desirably aimed at To show the practical value of the invention, large scale operation tests were carried out using electrolysis cells with and without hollow cathodes.

EXAMPLE I In an clectrolyzer having double-acting cathodes designed as shown in FIGS. 1, 2 and 3, a salt melt consisting essentially of MgCI- CaCl NaCl and KC] was electrolyzcd. The MgCl concentration was on an average about l5 7( by weight and the weight ratio (NaCl KCl)/CaCl was about 1.4. The molten bath temperature was 730C and the current density kA. On stabilization of operations, the following results were obtained:

4.9 V 0.46 amperes/em" 13.8 kWh/kg Mg Cell voltage Mean Current density Energy consumption Loss of chlorine from gas separation zone to metal separation zone U.(ll kg (l /kg Mg EXAMPLE 2 The test of Example I was repeated except that an electrolyzer was employed having conventional solid cathodes and having window openings in the partition wall at electrolyte surface level for the electrolyte to flow into the metal collection chamber. Results:

0.46 amperes/cm" 13.x kWh/kg M 0,: kg (mt M Cell voltage Mean Current density Energy consumption Loss of chlorine lytically active surfaces of separate of said alter nately arranged anodes; electrolysis zones between said facing cathode and anode surfaces; at least one gas separation zone positioned above and in communication with said electrolysis zones; a metal separation and collection zone separated from said electrolysis and gas separation zones; and passage means in each said cathode, spaced from said opposite electrolytically active surfacesthereof, for connecting the top of the respective said electrolysis zone with said metal separation and collection zone;

causing electrolyte, gas and metal to flow upwardly through said electrolysis zones;

causing said gas to continue to flow upwardly into said gas separation zone, while deflecting said elec' trolyte and said metal downwardly, in a direction opposite to the direction of flow of said gas, into said passage means within said electrodes;

passing said electrolyte and metal through said passage means into said metal separation and collection zone;

allowing said metal to rise and separate from said electrolyte in said metal separation and collection zone; and

returning said electrolyte to said electrolysis zones.

2. A process as claimed in claim 1, wherein said passage means comprises at least one duct in each said electrode, each said duct having an inlet opening in the top ofthe respective said electrode; and said step of defleeting comprises passing said electrolyte and metal downwardly through said inlet openings.

3. An electrolyzer for the molten salt electrolysis of metal chlorides, preferably for the molten salt electrolytic production of magnesium, said electrolyzer com prising:

alternately arranged anodes and cathodes of the double-acting type, each cathode having opposite exterior electrolytically active surfaces, said opposite surfaces of each cathode facing corresponding electrolytically active surfaces of separate of said alternately arranged anodes;

electrolysis zones between said facing cathode and anode surfaces;

at least one gas separation zone positioned above and in communication with said electrolysis zones;

a metal separation and collection zone separated from said electrolysis zones and said gas separation zone by a partition; and

passage means in each said cathode, spaced from said opposite electrolytically active surfaces thereof, for connecting the top of the respective said electrolysis zone with said metal separation and collection zone.

4. An electrolyzer as claimed in claim 3, wherein said electrolysis zones extend substantially vertically to said gas separation zone, the flow of electrolyte, gas and metal within each electrolysis zone being substantially vertically upwardly.

5. An electrolyzer as claimed in claim 4, wherein each said passage means comprises at least one duct extending through the respective said cathode and includes means for directing said electrolyte and metal substantially vertically downwardly in a direction opposite to the vertical upward direction of movement of said gas.

6. An electrolyzer as claimed in claim 5, wherein each said duct has an inlet opening at the top of the respective said cathode.

7. An electrolyzer as claimed in claim 6, wherein each said electrode comprises a double-walled hollow member with a U-shaped cross section, said member having an open top comprising said inlet opening, said member extending through said partition, each said duet having an outlet opening in a vertical plane of said partition.

8. An electrolyzer as claimed in claim 7, wherein each said member, at the portion thereof extending through said partition, has an inclined surface forming a ceiling of said outlet opening.

9. An electrolyzer as claimed in claim 7, wherein each said member has a bottom wall inclined towarc said partition and forming a bottom portion of the respective said duct. 

1. A PROCESS FOR THE MOLTEN SALT ELETROLYSIS OF METAL CHLORIDE , PREFERABLY FOR THE MOLTEN SALT ELECTROLYTIC PRODUCTION OF MAGNESI , SAID PROCESS COMPRISING: PROVING AN ELECTROLYZER INCLUDING ALTERNATELY ARRANGED ANODES AND CATHODES OF THE DOUBLE-ACTING TYPE, EACH CATHODE HAVING OPPOSITE ELECTROLYTICALLY ACTIVE SURFACES EACH FACING CORRESPONDING ELECTROLYTICALLY ACTIVE SURFACES OF SEPARATE OF SAID ALTERNATELY ARRANGED ANODES, ELECTROLYSIS ZONES BETWEEN SAID FACING CATHODE AND ANODE SURFACES, AT LEAST ONE GAS SEPARATION ZON POSITIONED ABOVE AND IN COMMUNICATION WITH SAID ELECTROLYSIS ZONES, A METAL SEPARATION AND COLLECTION ZONE SEPARATED FROM SAID ELECTROLYSIS AND GAS SEPARATION ZONES, AND PASSAGE MEANS IN EACH SAID CATHODE, SPACED FROM SAID OPPOSITE ELECTROLYTICALLY ACTIVE SURFACES THEREOF, FOR CONNECTING THE TOP OF THE RESPECTIVE SAID ELECTROLYSIS ZONE WITH SAID METAL SEPARATION AND COLLECTION ZONE, CAUSING ELECTROLYTE, GAS AND METAL TO FLOW UPWARDLY THROUGH SAID ELECTROLYSIS ZONES,
 2. A process as claimed in claim 1, wherein said passage means comprises at least one duct in each said electrode, each said duct having an inlet opening in the top of the respective said electrode; and said step of deflecting comprises passing said electrolyte and metal downwardly through said inlet openings.
 3. An electrolyzer for the molten salt electrolysis of metal chlorides, preferably for the molten salt electrolytic production of magnesium, said electrolyzer comprising: alternately arranged anodes and cathodes of the double-acting type, each cathode having opposite exterior electrolytically active surfaces, said opposite surfaces of each cathode facing corresponding electrolytically active surfaces of separate of said alternately arranged anodes; electrolysis zones between said facing cathode and anode surfaces; at least one gas separation zone positioned above and in communication with said electrolysis zones; a metal separation and collection zone separated from said electrolysis zones and said gas separation zone by a partition; and passage means in each said cathode, spaced from said opposite electrolytically active surfaces thereof, for connecting the top of the respective said electrolysis zone with said metal separation and collection zone.
 4. An electrolyzer as claimed in claim 3, wherein said electrolysis zones extend substantially vertically to said gas separation zone, the flow of electrolyte, gas and metal within each electrolysis zone being substantially vertically upwardly.
 5. An electrolyzer as claimed in claim 4, wherein each said passage means comprises at least one duct extending through the respective said cathode and includes means for directing said electrolyte and metal substantially vertically downwardly in a direction opposite to the vertical upward direction of movement of said gas.
 6. An electrolyzer as claimed in claim 5, wherein each said duct has an inlet opening at the top of the respective said cathode.
 7. An electrolyzer as claimed in claim 6, wherein each said electrode comprises a double-walled hollow member with a U-shaped cross section, said member having an open top comprising said inlet opening, said member extending through said partition, each said duct having an outlet opening in a vertical plane of said partition.
 8. An electrolyzer as claimed in claim 7, wherein each said member, at the portion thereof extending through said partition, has an inclined surface forming a ceiling of said outlet opening.
 9. An electrolyzer as claimed in claim 7, wherein each said member has a bottom wall inclined toward said partition and forming a bottom portion of the respective said duct. 